Effects of Exercise on Types of Muscle Fibers

Skeletal muscles are made of a variety of fibers. The composition of your muscles determines how they will perform, whether your activity involves sprinting or endurance work. Most people's muscles contain more of one fiber type than another; therefore, you may be better suited to one particular type of exercise.

Slow-Twitch Fibers

Slow-twitch muscle fibers are slower to contract, but they take a long time to fatigue. This makes them ideal for endurance work. In fact, they play a role in many of our activities of daily living, such as walking, housecleaning and maintenance of your posture.

Fast-Twitch Fibers

Fast-twitch muscle fibers contract quickly, but they also fatigue quickly. These fibers are well-suited for short burst activity, such as sprinting, hurdling and jumping. In addition, fast-twitch fibers generate more force than slow-twitch, so a muscle containing more fast-twitch fibers can lift a heavier load. In the limb muscles, most people have predominantly fast-twitch fibers, since you need more power for movement. Some individuals have a greater percentage of fast-twitch than others.

Increasing Fast Twitch Character

No matter how much of one type of exercise you do, you can't change your predominantly slow-twitch muscles to become fast twitch. However, through specific training, you can change the size of the fibers, changing the proportion of each type in the muscle. For instance, if you trained your slower-twitch muscle on a strength program, while the slow-twitch fibers would not convert to fast-twitch, the fast-twitch fibers would increase in diameter, and the slow-twitch fibers would decrease in diameter due to lack of specific training. In this sense, you can increase the proportion of fast-twitch muscle tissue.

Training For Strength Gains

To increase the proportion of fast-twitch tissue in your muscles, you'll need to train specifically. In the weight room, this consists of a short set of up to eight repetitions using a heavy weight. Using light weights and performing many repetitions trains the slow-twitch fibers. If you are a runner, performing frequent sprint workouts does not appear to change your muscular proportions. Performing sprint workouts in combination with heavy strength exercises for your legs may show an improvement.